Field of the Invention
The present invention concerns an examination of an examination subject using a magnetic resonance (MR) facility based on a fingerprinting measurement.
Description of the Prior Art
In clinical imaging, MR images usually display only a qualitative contrast. The exact pixel values are subject to many influences, such as the parameter settings selected for measurement (e.g. TE, TR, range) and factors that cannot be influenced by the user (e.g. coil sensitivity, software versions, scanner type).
With many applications, a so-called quantitative MR image would be desirable in which the pixel values would correspond to “genuine” physical variables (e.g. the T1 relaxation time, the T2 relaxation time, the off-resonance, the proton density). So-called MR fingerprinting, which is described in “Magnetic Resonance Fingerprinting”, Ma inter alia, Nature 2013 Mar. 14; 495(7440): 187-192. doi:10.1038/nature11971, is one such quantitative MR imaging method. With MR fingerprinting numerous measurements are performed, wherein measurement parameters or recording parameters (e.g. flip angle, TR (Time to repetition), TE (Echo Time), TI (Inversion Time), an embodiment and/or a number of RF pulses, an embodiment and/or a number of gradient pulses, diffusion coding) are varied pseudo-randomly. The MR signal for each voxel is determined for each measurement, thus producing an MR waveform characteristic of the voxel and/or pixel for each voxel and/or pixel, which may be regarded as a fingerprint. With the use of a database, such a fingerprint may be assigned to a certain n-tuple of physical values (e.g. T1 relaxation time, T2 relaxation time, off-resonance, proton density), and thus to a certain substance (e.g. CSF, gray matter, fat).
MR waveforms are stored in the database for many of these n-tuples of physical values for the pseudo-random variation of the recording parameters. Usually, these stored MR waveforms are generated by a simulation with the use of Bloch equations depending on the pseudo-random variation of the recording parameters. In order, for example, to have an MR waveform for each combination of a T1 relaxation time in a range of 100 ms to 5000 ms and a T2 relaxation time in a range of 10 ms to 2000 ms with a resolution of 10 ms, almost 100,000 MR waveforms must be available.
In order for the MR waveform or fingerprint characteristic of each voxel and/or pixel to be sufficiently meaningful, a large number (up to 5000) of MR images is acquired for the generation of this MR waveform. According to the prior art, so-called single-shot MR methods are used with which, for each repetition, i.e. based on only one RF excitation pulse, the entire MR image is acquired, i.e., the entirety of k-space is filled. Usually a spiral k-space trajectory with high undersampling is employed. Accordingly, the MR images that are created in the process are very heavily overlaid by artifacts, which may be ascribed to undersampling but may also be due to system inaccuracies of the MR scanner.